This invention relates to a process for production of microcellular or porous cast resin products utilizing water-in-oil emulsioss of polymerizable components. The process of the invention achieves complete polymerization of the resin components, thereby minimizing shrinking and warping of the cast final product and providing improved strength, throughout a wide range of water contents in the emulsions. This is accomplished by placing the casting in a fluid medium maintained at a temperature and pressure such that evaporation of water and polymerizable components is inhibited but sufficient to promote continued and complete cross-linking of the polymerizable components.
While not so limited, the process of the present invention has particular utility in the fabrication of picture frames, reproductions of bas relief or sculptured art objects and similar decorative items, furniture parts and overlays, and structural members of relatively high strength and rigidity. Castings made in accordance with the invention can be cut, machined and nailed.
U.S. Pat. No. Re. 27,444 (original U.S. Pat. No. 3,256,219) discloses a process for the production of porous resin products which comprises forming a water-in-oil emulsion composed of an aqueous medium (as the dispersed phase), and (as the continuous phase) a polymerizable organic liquid containing at least one ethylenically unsaturated group and a substantially water-insoluble polymeric compound acting as an emulsifier, polymerizing the organic liquid without breaking the emulsion, thereby forming a solid resin material having water droplets dispersed therein, and drying, either at elevated temperature, in a vacuum, or allowing it to stand in a current of air.
This patent indicates that the aqueous phase remains in dispersed state after the monomers are polymerized by use of microgels acting as emulsifiers. Preferred polymerizable organic liquids include vinyl esters, acrylic acid esters, methacrylic acid esters, styrene, acrylonitrile and unsaturated hydrocarbon halides.
The preferred polmeric compound acting as an emulsifier may include polymerizable carboxylic acids, such as acrylic acid, an unsaturated polyester, polystyrene, and polyvinyl esters. When polystyrene is used as the emulsifier, a molecular weight of at least 10,000 is preferred. The water content of such emulsions is at least 25% by weight and preferably may range from at least about 40% by weight to not less than about 70% by weight.
U.S. Pat. No. 4,077,931, assigned to Ashland Oil, Inc., discloses thermosetting water-in-oil emulsions comprising water as the dispersed phase, unsaturated polyester having a mean molecular weight ranging from 1,800 to 100,000, the polyester being the reaction product of polycarboxylic acid, at least a portion of which is .alpha., .beta. ethylenically unsaturated, and a polyhydric alcohol, and an ethylenically unsaturated solvent for the polyester which is copolymerizable therewith. The weight ratio of polyester and solvent to water is allegedly in the range of 1:10 to 10:1, while the weight ratio of polyester to solvent is in the range of 10:1 to 1:10.
The polyesters of this patent are formed from polycarboxylic acids of 4 to 18 carbon atoms such as malonic acid, adipic acid, succinic acid, phthalic, isophthalic, and terephthalic acids, fumaric acid or maleic acid. Acid anhydrides may also be used. These are reacted with polyhydric alcohols such as ethylene glycol, diethylene glycol, propylene glycols and the like.
Solvents which are copolymerizable with the polyester include styrene, vinyl toluene, acrylonitrile, acrylate and methacrylate esters, vinyl esters and the like. These are in monomeric form in the emulsion.
A peroxide catalyst, cobalt compounds and a tertiary aromatic amine are also present as polymerization promoters and initiators.
The emulsions of U.S. Pat. No. 4,077,931 are alleged to be stable and, when cast into a mold, polymerize or cure partially by an exothermic reaction without phase separation to a product having a microcellular structure.
Bulletin 1437 entitled "Aropol WEP.RTM. Resins", published in 1980 by Ashland Chemical Company (a division of Ashland Oil, Inc., the assignee of U.S. Pat. No. 4,077,931) describes water-extended casting resin emulsions of two general types. One type is that disclosed in U.S. Pat. No. 4,077,931 which produces a so-called closed cell foam, while the other type is that disclosed in U.S. Pat. No. Re. 27,444 producing a so-called open cell foam. While a wide range of emulsion water contents is alleged to be operative in these two patents, Bulletin 1437 clearly indicates that the water content must be controlled within the range of 50% to 60% by weight. Even an increase in water content from 50% toward 60% in the closed cell type is stated to produce reduced strength, reduced peak exotherm temperature, increased emulsion viscosity, increased gel time, and increased water loss rate. The only counterbalancing advantage is reduced cost. There is no indication that more than 50% water can be used in the open cell type.
According to Bulletin 1437, when producing closed cell type products the part is permitted to reach the peak exotherm temperature after casting and is then removed from the mold. A shrinkage of about 1% to 2% occurs during curing in the mold. The casting is next permitted to cool to room temperature and is stated to be about 90% cured in one hour. The product is then permitted to stand for 24 hours at ambient temperature, prior to painting. Thereafter, further long term shrinkage occurs, amounting to about 2% after 2 years, accompanied by a weight loss of about 12%.
When producing open cell type castings the product ". . . will continue to shrink until the final dehydrated part is 4% to 5% smaller than the mold." Open cell castings should be dehydrated, according to Bulletin 1437, by heating in an oven maintained at 150.degree. to 200.degree. F. Castings dehydrated at room temperature and subjected to elevated temperatures later ". . . will relax to their final size of approximately 95% of the mold size."
It is thus alleged that there is about 1% difference in shrinkage when subjected to oven heating (about 4% to 5%) as compared to dehydration at room temperature and heating at a later time (about 5%). This indicates some further, but still incomplete, curing or cross-linking of the resin when oven heating is conducted immediately after cooling to room temperature.
Bulletin 1437 states that since shrinkage and warpage of closed cell type products are unavoidable and variable, the cast products cannot be used in applications in which dimensional stability is critical, such as in furniture parts.
Product bulletins published by Reichhold Chemicals, Inc. in August 1984 describe polyester resin compositions intended for use as water filled castings. A rigid resin composition (closed cell type designated Polylite.RTM. Polyester Resin 32-183) and a resilient resin composition (closed cell type designated Polylite.RTM. Polyester Resin 32-182) are stated to contain more than 40% unsaturated orthophthalic polyester resin and from 56% to 60% styrene monomer. After mixing with water with vigorous agitation and addition of 1% to 2% methyl ethyl ketone peroxide catalyst, a moderate exothermic cure is alleged to be obtained with an emulsion containing 50% by weight water. After casting the recommended procedure is an overnight cure at room temperature and a post-cure for two hours at 121.degree. C. (250.degree. F.). The bulletins state that the cured water-filled castings will in time lose some entrapped water and hence do not maintain dimensional stability. Accordingly, the resin compositions are not recommended ". . . for use in castings or parts which must maintain a high degree of dimensional stability."
It is to be noted that the Ashland publication (Bulletin 1437) indicates that substantially greater shrinkage occurs with open cell than with closed cell castings.
U.S. Pat. No. 2,707,804 discloses apparatus for steam curing foamed latex, comprising a mold having hollow cores projecting into the mold at selected points, each core having an apertured wall through which dry superheated steam is discharged into the latex foam surrounding the cores, at a pressure of less than 2 psig. The latex is cured by the dry steam. The apparatus is alleged to be effective in curing a cellular latex mass 2 inches thick in about 5 minutes and a cellular latex mass 4 inches thick in about 15 minutes. The steam supplied to the mold is preferably superheated to about 300.degree. F., but is permitted to expand with consequent reduction in pressure and is passed through a filter for removal of condensed water so that the pressure when entering the hollow cores does not exceed 2 psig. The temperature of the dry steam at this point is not disclosed but is of course less than 300.degree. F. The use of dry steam is necessary for natural or synthetic latices since such a latex is an oil-in-water emulsion. Wet steam would cause the latex to sag in the mold since excessive water would be introduced into the open cell latex foam.
U.S. Pat. No. 4,384,047 discloses a process for producing an ultrafiltration membrane of a vinylidene fluoride polymer, or copolymer thereof with tri- or tetrafluoroethylene. Minor amounts of methyl methacrylate can also be included. A solution of the polymer components in an organic solvent such as triethyl phosphate, hexamethyl phosphoramide, or 1-methyl-2-pyrrolidinone, is formed with the solvent ranging from about 65 to about 80% by weight. From about 1% to 10% by weight of a non-solvent is added, such as glycerol, ethylene glycol, or phosphoric acid, and the liquid mixture is cast on a smooth solid surface to form a thin (10 to 20 mils) sheet. A portion of the solvent and non-solvent is then evaporated from the sheet, preferably at ambient temperature, and the sheet is next contacted with a liquid gelation medium maintained at a temperature ranging from about -10.degree. to about +50.degree. C., preferably 0.degree. to 5.degree. C. A stabilizing treatment may later be conducted which involves immersing the sheet in warm water or heating in an oven, at a temperature ranging from about 20.degree. to about 100.degree. C., with maximum shrinkage occurring after about 30 minutes at 100.degree. C. The resulting membrane is stated to have a pore size range suitable for ultrafiltration, with a surface layer containing pores having a diameter ranging from about 1 to about 100 millimicrons and a support layer thicker than and having less resistance to fluid flow than the surface layer.
In this method the purpose of the liquid gelation step is to prevent wrinkling of the cast film, which is not physically restrained. In the specific examples the membrane or film floated free of the casting surface during the gelation step. The optional heat stabilization may be conducted before or after drying of the membrane, the stated purpose being to avoid shrinkage when the membrane is used to filter heated fluids. Actually, the heat stabilization results in pre-shrinking.
Other prior art relating to porous resin products includes U.S. Pat. Nos. 3,734,867; 3,975,348 and 4,522,953.
Despite efforts by several large corporations over a period of years to develop water-in-oil resin emulsions and processes for economical production of porous, microcellular cast resin products, no solution has yet been discovered for the problem of shrinkage and warpage in low cost cast products. Accordingly, these prior art products cannot be used in applications wherein dimensional stability in the as-cast condition is critical, thus excluding a potentially large market in such fields as furniture parts.